Various systems currently exist, which assist in guiding and navigating a surgical probe through a patient undergoing a surgical procedure. These systems include, for example, fluoroscopic, ultrasonic, conductive, optical and electromagnetic type navigation systems.
Various electromagnetic type navigation systems have already been proposed, such as that disclosed in U.S. Pat. No. 4,821,731 to Martinelli, et al., U.S. Pat. No. 5,592,939 to Martinelli and U.S. Pat. No. 5,913,820 to Bladen, et al., which are each hereby incorporated by reference. Advantages of such electromagnetic tracking navigation systems are significant over existing navigation systems. For example, low-frequency electromagnetic waves are not attenuated by the body and therefore, there are no “line-of-sight” issues as with existing optical systems. The transmitter coil array may also be placed underneath or above the patient and the navigated surgical instrument or probe may be used above or below the transmitter coil array. The receiver coils utilized in the surgical instrument or probe are also generally much smaller than existing type navigation systems, which may enable surgical procedures that were previously impossible due to instrument size. The small size of the receiver coils also enable the receiver coils to be placed near the tip of the instrument providing further accuracy and the ability to navigate non-rigid instruments.
However, electromagnetic type navigation systems do have the disadvantage that the electromagnetic field may be distorted by metal objects, sometimes referred to as metallic distortions. In this regard, metal objects that are generally large in size cause the magnetic field to bend, thereby possibly causing inaccuracy in the reported probe position. The other effect of positioning a metal object near the electromagnetic field being navigated is conduction effects. For example, a metal object positioned near or in the electromagnetic field, such as a fluoroscope (C-arm) or an OR table, may create a virtual coil along the metal surface that creates an interference back into the magnetic field. Again, this may create an inaccuracy in the reported probe position because the exact field strengths in the previously known electromagnetic fields have been altered due to the metal object.
In order to reduce or eliminate the effects of distortion due to metal objects, known mathematical models of the electromagnetic fields produced by the transmitter coil array may be utilized. If these mathematical models are accurate, they can be used to represent a set of “known” fields used during the navigation process. However, the disadvantage with using mathematical models for the transmitted fields is that there are inherent inaccuracies in the manufacturing process of the transmitting coils in the transmitting coil array, which can lead to incorrect field values, which are mathematically modeled. These incorrect field values may lead to inaccuracy in the overall navigation process. The mathematical models are also generally very mathematically complex and may, therefore, take an unreasonable amount of time for a computer to calculate and process.
What is needed then is a method and apparatus for electromagnetic navigation of a surgical probe near a metal object, which does not suffer from the above-mentioned disadvantages. This will, in turn, provide electromagnetic navigation of a surgical probe near a metal object that has greater accuracy, provide a shield to reduce or eliminate the effects of the metal object, provide a universal connection to connect the shield to the metal object, provide a calibration process that takes into effect either the shield or the metal object, provide a set of transmitting coils, which may be attached to the shield, integrated into the shield or integrated into the metal object itself, and provide wireless communications in the electromagnetic navigation system for ease of assembly into existing hardware. It is, therefore, an object of the present invention to provide such a method and apparatus for electromagnetic navigation of a surgical probe near a metal object.